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Condition Index and Shell Pests

Six oyster shells of various sizes and coloring are positioned in a row on a black surface.

Aside from the oysters collected for reproduction and disease monitoring additional oysters are collected for Condition Index processing. Condition Index has been used as a monitoring tool for analyzing how water pollutants affect the condition of oysters by comparing the dry tissue weight of the animal to the interior volume of the shell. Outside of available nutrients in the water CI is largely dependent on temperature, salinity, and reproductive stage. The presence of parasites in oysters can inhibit the amount of food that they receive, which can impact their energy levels. Lower energy levels can negatively affect growth and reproduction of the oysters and can result in a lower condition of the oyster.

Oyster reefs also provide a complex 3-dimensional structure which results in numerous microhabitats as well. Organisms such as the mud worm Polydora spp., boring clam Diplothyra spp., and boring sponge Cliona spp., utilize the oysters shell as habitat. Shells from oysters that were dissected for Condition Index processing are kept for shell pest analysis. Pictures of the exterior and interior of each oyster shell (top and bottom) are taken and then uploaded to an image analysis software where biologists can quantify the damage done by pests.

The mudworm, Polydora spp., has a higher prevalence in areas with higher salinities and higher temperatures. Periods of cold temperatures and large freshwater influences can have a negative effect on the mudworm. Polydora spp. burrows inside of the oyster and creates tubes in the shell in an attempt to create a barrier between itself and the parasite, the oyster will create a shell layer over the mudworm infestation.

Like Polydora spp., the boring clam Diplothyra spp. and boring sponge Cliona spp., are both influenced by water temperature and salinity regimes. Higher temperatures and salinities can increase the prevalence of both of these organisms. Cliona spp. is usually a yellow-ish color, but can also be other colors like brown, orange, purple, or red. This sponge chemically creates small holes in the shell of oysters and other bivalves where it grows out from the holes. Boring clam, Diplothyra spp., bores inside the oyster shell where it lives out its life as a commensal organism.